The present invention relates to a method for manufacturing an aluminum alloy fin material for brazing, using a twin-roll-type continuous cast-rolling method (or abbreviated as a continuous cast-rolling method) and cold-rolling.
A heat exchanger made of an aluminum alloy, such as a radiator, assembled by brazing, has a corrugated fin 2 integrated between flat tubes 1, as shown in FIG. 1, and both ends of the flat tube are open to the spaces formed by a header 3 and a tank 4. A heated refrigerant is sent into the flat tube 1 from one of the tanks, and the cooled refrigerant, by heat exchange at the part of the flat tube 1 and fin 2, is collected into the other tank, to be recirculated.
An extrusion flat tube having multi-pore, a plate manufactured by press-molding of a brazing sheet, in which a core material is clad with a sheath material (such as a brazing material of an Alxe2x80x94Si-series alloy), or an electro-seam welding flat tube, is used for the above-described tube 1. A fin comprising a brazing sheet manufactured by cladding the sheath material onto both surfaces of the core material, or a fin comprising an Alxe2x80x94Mn-series alloy (such as 3003 alloy or 3203 alloy) excellent in buckling resistance, is used for the above-described fin.
Since the heat exchanger has been required to be of small size and light weight in recent years, the fin material constituting the heat exchanger tends to be thin. Consequently, the fin material is emphasized to have improved mechanical strength, because the fin may collapse during assembly of the heat exchanger, or the radiator may break during use when the mechanical strength of the fin material is insufficient. In addition, improvement of heat conductivity of the fin material itself has been required, since the amount of heat transport of the fin material is thought to be important as a result of thinning of the fin material in response to the small size and light weight of the heat exchanger, such as a radiator.
However, the conventional Alxe2x80x94Mn-series alloy fin material has the problem that an increased Mn content, to enhance the mechanical strength of the fin material, leads to a large decrease in heat conductivity. On the other hand, an increased Fe content results in crystallization of a large quantity of intermetallic compounds, which works as recrystallization nuclei when the fin material recrystallizes by brazing, to form fine recrystallization textures. Since this fine recrystallization texture involves many crystal grain boundaries, a problem is caused that the brazing material diffuses along the crystal grain boundaries during the brazing step, thereby decreasing the droop resistance of the fin material.
An Alxe2x80x94Fexe2x80x94Ni-series alloy fin material (JP-A-7-216485 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application), JP-A-8-104934, and the like), which is proposed other than the above-described Alxe2x80x94Mn-series alloy fin material, is excellent in mechanical strength and heat conductivity. However, the alloy is not suitable for thinning, because self-corrosion resistance of the fin material itself is lowered.
Several fin materials according to the manufacturing method by continuous cast-rolling and cold-rolling have been proposed, since the method requires low plant investment. For example, an Alxe2x80x94Mnxe2x80x94Si-series alloy fin material (JP-A-8-143998) has been proposed, to prevent fatigue strength from decreasing, wherein primary crystal Si is allowed to localize at the center in the direction of thickness, by continuous cast-rolling and cold-rolling, and recrystallized grains are coarsened by preventing the primary crystal Si from working as recrystallization nuclei, thereby suppressing invasion of the brazing material into the crystal grain boundaries.
Other examples include an Alxe2x80x94Mnxe2x80x94Fexe2x80x94Si-series alloy fin material (WO 00/05426), in which mechanical strength and electrical conductivity are enhanced by prescribing the cooling rate in the continuous cast-rolling; and an Alxe2x80x94Mnxe2x80x94Fe-series alloy fin material (JP-A-3-31454), in which brazing properties are improved by removing an oxidation film, formed by continuous cast-rolling, by alkali cleaning before or during the cold-rolling step.
However, most of Si has been crystallized as the primary crystal Si during the casting step in the invention disclosed in the above-described JP-A-8-143998. Consequently, the material may break during the rolling step, by forming the primary crystal Si that works as initiation points, or the fin material may break during the corrugation process. The thinner fin material is more readily broken during the corrugation process, and sometimes the fin material cannot be machined at all. In these cases, since the amount of Si incorporated into crystallized materials is small, to cause a depletion of crystallization nuclei (an Alxe2x80x94Fexe2x80x94Mnxe2x80x94Si-series intermetallic compound) in the intermediate annealing step, or since precipitation of the intermetallic compound is further suppressed without hot-rolling or batch-type intermediate annealing step, the amount of Mn in the solid solution increases, to result in decreased heat conductivity. Further, since Si is segregated at the center of the fin material, the fin material becomes poor in fin melt resistance.
While the object of the invention in the above-described WO 00/05426 is to enhance precipitation by forming Mn-series fine intermetallic compounds, and to improve heat conductivity by precipitating Mn, a sufficient precipitation-enhancing effect has not been obtained, due to a smaller Mn content as compared with the present invention. When the Mn content is increased, to enhance precipitation, a coarse Mn-series compound (Alxe2x80x94Fexe2x80x94Mnxe2x80x94Si compound) is precipitated, to decrease the corrugate formability. Since this fin material has a crystal grain diameter of as small as 30 to 80 xcexcm after brazing, the fin melt resistance of the fin material decreases by diffusion of the brazing material. Furthermore, an Alxe2x80x94Fexe2x80x94Si-series compound, as a cathode site, precipitates due to a small content of Mn, it decreases the self-corrosion resistance of the fin material itself.
The alloy composition of an invention in the above-described JP-A-3-31454 overlaps the composition of the present invention, either when the invention includes Si, or when the invention includes Si as well as any one of Cu, Cr, Ti, Zr or Mg. However, according to the method disclosed in the above-described publication, a Alxe2x80x94Fexe2x80x94Mnxe2x80x94Si-series fine compound cannot be precipitated, even though the brazing ability of the fin material may be improved. Resultantly, various properties required for making the heat exchanger small in size and light in weight have not been satisfied.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.